Triode sputtering apparatus and method using synchronized pulsating current



United States Patent O i 3,464,907 TRIODE SPUTTERING APPARATUS ANDMETHOD USING SYNCHRONIZED PULSATING CURRENT John G. Froemel, Verona, andMeyer Sapoff, West Orange, NJ., assgnors to Victory EngineeringCorporation,

Springfield, NJ., a corporation of Delaware Filed Feb. 23, 1967, Ser.No. 617,998 Int. Cl. C23c 15 00 U.S. Cl. 204-192 13 Claims ABSTRACT FTHE DISCLOSURE A device and method for depositing molecules of a desiredmaterial on a substrate or base is described. Deposition is accomplishedin a rarified atmosphere containing a noble gas. The initial dischargeis created between an anode and a cathode which creates electrons andgas ions. An auxiliary target of the material to be deposited is biasedwith respect to the anode and attracts the gas ions which ions causeemission of the desired molecules. A substrate is mounted parallel tothe target and the molecules are deposited on it. Both the maindischarge between the anode and cathode and the auxiliary dischargebetween the anode and the target are pulsed by the application of highvoltage.

Summary of the invention This invention relates to an apparatus fordepositing molecules on a substrate to form a thin film such as asemiconductive film. The invention has particular relationship to theapplication of high voltage pulses which are applied to both the main orplasma initiating discharge and the auxiliary discharge which transfersthe molecules from the target to the substrate, however, the secondarydischarge is one which is biased between the anode and the cathode. Onefeature of the invention is the use of a target in the form of a blockof material to be deposited or a porous plug to one surface of which agas, vapor, or liquid is conducted. Where the target is a gas, as thegas molecules are drawn through the electron discharge beam, they arepolymerized to form a solid polymer compound. This compound is depostedon the substrate. Another feature of the invention includes the use of afiled emission cathode which employs a hot filament and a pointeddischarge element often called a field emissioncathode or point. Anotherfeature of the invention includes the use of a magnetic field which maybe movable to distribute or sweep the molecules evenly on the substrate.

Depositing molecules on a substrate by the use of a gas discharge is oldin the art. While direct currents and alternating currents have beenused for creating these discharges, it has been found that theapplication of sharp voltage pulses to the discharge electrodes producesan additional and unexpected rate of deposition, furthermore, thismethod also decreases the amount of heat generated during suchdeposition. This increased deposition is even more pronounced when themain discharge between the anode and cathode is turned on first and thena short time later the voltage between the target and the anode isapplied. The target pulse is stopped before the plasma pulse is turnedoff. This method of pulsing produces a marked increase in depositionrate and lowers the integrated target current thu affording a reductionin the amount of heat generated.

Description of the drawings FIGURE l is a cross-sectional view of theapparatus showing the anode, the cathode and the target. This view istaken along line 1-1 of FIGURE 2.

3,464,907 Patented Sept. 2, 1969 ice l FIGURE 2 is a cross sectionalView of the apparatus shown in FIGURE l, and is taken along line 2-2 ofthat figure. This view also shows the substrate.

Referring now to FIGURES 1 and 2, the device includes a ring-shaped base10, on which a bell jar 11 is placed. The base 10 is connected to theconduit 12 of a pumping system (not shown). A plurality of electrodes22, 51 and S2 are led into the base 10. The pumping system (not shown)must be particularly efiicient in order to remove all traces of absorbedgasses and other undesirable hydrocarbon compounds from the interiorof'the bell jar 11. Such a pumping system may consist of either or allof a getter tantalum sublimation pump, a Vac-Ion pump or a cryogeneticabsorption pump. In order to eliminate all undesirable substances fromthe bell jar, and associated pumping system, the entire apparatus issubjected to a temperature of at least 250 C. This may be done by localheating means or a furnace hood may be lowered over the entire systemand heated in the usual manner. This part of the apparatus is not shownin the drawing because such equipment is old in the art and has beendescribed before in books and other publications.

A field emission cathode 20 is carried within a conduit 13, mountedinside the bell jar 11. The field emission cathode 20 is in the form ofa pointed tip 20, which is preferably made of tungsten. An associatedfilament 14 which is indirectly heated supports the cathode 20.Electrical insulation between the filament 14 and the conduit 13 isprovided by a glass spacer 16. The lead-in wires 15, 15a, to thefilament 14 are passed through a metal-to-glass seal 16a, and areconnected to a source of current 17 which may be a battery or other DCpotential source.

Because of the pulse nature of the discharge the pointed cathode wire 20is welded to the top of the filament 14. The wire 20 increases theelectric field at its tip and thereby pemits the discharge to start witha rapid rise time. The shield 13 prevents direct evaporation of thetungsten tip 20 and filament 14 from reaching or contaminating thesystem.

An anode 21 is mounted in the bell jar 11, opposite the conduit 13, andis preferably made with a concave stainless steel surface. The anode 21is supported on a vertical rod 22 which is also brought out through ahermetic seal 23, carried by a portion of the base 10. The end of therod 22 acts as an electrode for the application of a high voltage pulsewhich is derived from an external power source. The external powersource may be any source of pulsed potential such as the pulse amplifier24, and generator 40. The pulse width and pulse repetition rate may bevaried by means of the accompanying generator 40 for the power supply.The negative terminal of the source 24, 40, is connected to one of thecathode leads 15, which is grounded to create the main discharge. Thisin turn creates the desired plasma, when a gas such as argon or xenon isadmitted into the bell jar 11 by means of the variable leak valve 53.This gas is preferably maintained at a pressure in the bell jar of below10 microns.

A target 26 is carried by a combined conduit and support 27, whichextends downwardly to the base and is supported by a portion of the base10 through a hermetic seal or insulator 50. The target 26 includes ablock 30 which may be a block of semiconductor material, metal or othersolid material such as an insulator that is desired to be deposited onthe substrate 35 across from it. Many forms of oxide and many types ofmixtures of metals and oxides can be sputtered from the target to thesubstrate and any of these mixtures can be used for this apparatus.

Alternately, the target 26, as shown in the figures, may be in vaporform. By substituting for the block 30 a porous material, which has beenformed by sintering, it is possible to direct a monomer gas therethroughas shown in FIGURES l and 2. The monomer gas is led through the conduitsupport 27 and then through the block. Porous porcelain or porous glassmay be used instead of a sintered metallic powder for the block 30 inthis embodiment. The porous block 30 is held by frame work 31 whichgrips the block on its sides but provides a space 32 at the rear of theblock. This space 32 is in communication with the interior 33 of theconduit 27. When it is desired to deposit the polymer of a monomer onsubstrate 35, a monomer gas supply is connected to conduit 27 andpermitted to slowly diffuse through the porous plug 30.

In order to provide a bias for an electrical charge, a grid or porouselectrode 34 is placed on the front surface of the porous block 30.These wires 34 may be made of platinum or other noble metals of the hightemperature class or any conductive material which sputters less easilythan the target material. The electrode 34 is connected to the pulsepower supply 40, 54.

The substrate 35 provides the support on which the semiconductormaterial or polymer is deposited. The substrate is supported by a rod 36similar to rod 22 which is led out through an insulating connection 52.This substrate support may or may not be connected to ground potential,depending upon the material to be sputtered. It has been found that thedeposition of molecules from the target 26 is not always spreaduniformly over the surface of the substrate 35 or, spread completelyover the entire surface when larger areas are to be covered. In order tomake a deposit more uniform and complete a permanent magnet orelectromagnet 41, may be installed inside the bell jar 11, and arrangedfor rotation and motion about a pivot axis 42. It is well known thatmoving ions will be deflected by a magnetic field. The amount ofdefiection and exact position of the magnet are functions of the ionsand the voltages used to generate the discharge. The best arrangementfor the magnet field for each material must be determinedexperimentally.

The operation of this device will be apparent from the foregoing. Thesubstrate 35 is placed in its holder and the bell jar 11 slipped uponits base 10. The bell jar 11 may be either constructed from glass,stainless steel or similar metal. The vacuum pumps (not shown) arestarted and the interior space of the bell jar cleaned and fiushed. Theentire system is then baked out by the application of exterior heat ofthe order of 250 C. After these preliminary operations an inert gas suchas argon is introduced into the evacuated bell jar at a pressure ofmicrons of mercury. This gas is ionized by the discharge between thecathode 20, 14, and the anode 21.

The material to be sputtered in this case a solid block 30, which hasbeen placed in the holder 33, and may consist of a metal semiconductoror insulator is then biased negative with respect to the anode 21.Positive argon ions are accelerated to the target 26 and cause thetarget molecules to be sputtered onto the substrate 35.

The operation of this device for depositing a polymerized materialincludes the insertion of the substrate 35 into its holder andpositioning of the bell jar 11 onto its base 10. The vacuum pumps arestarted and the interior space of the bell jar is cleaned and flushedand the entire system baked out while under the application of exteriorheat of the order of 250 C. After these preliminary operations an inertgas such as argon is put into the evacuated space at a pressure of about10 microns of mercury.

If a polymer is to be applied to the substrate 35, a monomer gas isapplied to conduit 27 and diffused through the porous plug 30. The pulsegenerator which biases the target at a negative potential with respectto .4 the anode is started, however, pulses are first applied to theanode and cathode. After the material has been deposited to the requiredthickness, the discharges are stopped and the gas flow to conduit 27 isturned oif. The gas which generates the plasma is also turned off andthe bell jar is thoroughly evacuated of all gas. A neutral gas may alsobe used to backfill the bell jar if desired. However, in most instancesair is admitted to the system and the substrate detached from itsmounting.

Having thus fully described the invention, what is claimed as new anddesired to be secured by Letters Patent of the United States is:

1. Apparatus for depositing a thin film of material on a substratecomprising a sealed envelope connected to a pumping system forexhausting and filling the envelope with an inert gas at reducedpressure, a cathode within said envelope having sealed lead-inconductors for connection to a first external circuit, an anode withinsaid envelope having a sealed lead-in conductor for connection to asecond external circuit, said anode and cathode bracketing a dischargepath therebetween, a target within the bell jar adjacent to thedischarge path, said target supporting the material for deposition, afirst Source of electrical power for applying a series of positivevoltage direct current pulses to said anode to produce a pulse dischargebetween the anode and cathode, a second source of electrical power forapplying a series of negative voltage direct current pulses to thetarget for attracting the ions which liberate the material from thetarget, and a subsrate within the bell jar opposite to said target onthe other side of the discharge path for receiving material from thetarget.

2. Apparatus as claimed in claim 1 wherein said cathode consists of athermal emissive filament and a pointed electrode connected to saidfilament for the production of high intensity electric iield and plasma.

3. Apparatus as claimed in claim 1, wherein said target includes aporous plug, a source of gas molecules in communication with the insideof said envelope adjacent the porous plug, whereby the gas ispolymerized by the discharge between the anode and cathode.

`4. Apparatus as claimed in claim 3 wherein a plurality of conductorsare spaced on the surface of the porous block in the path of the gas.

5. Apparatus as claimed in claim 1 wherein the first and second sourcesof electrical power are derived from a pulse generator.

6. Apparatus as claimed in claim 1 wherein said first and second sourcesof electrical power are derived from direct current power sourcesconnected in series with electronic switching means.

7. Apparatus as claimed in claim 1 wherein a source of magnetic flux isapplied to said discharge between the cathode and anode to distributeand deposit molecules evenly on a substrate,

8. A process of depositing material on a substrate comprising thefollowing steps of creating a direct current pulsed gaseous dischargebetween an anode and cathode within a sealed envelope, positioning atarget containing material to be deposited adjacent to said discharge,positioning a substrate on which the material is to be depositedadjacent to said discharge but on the side opposite the target andapplying negative voltage direct current pulses to the target forcreating a pulse electric field between the target and the anode.

9. A process as claimed in claim S, wherein in said electric pulsesapplied to the anode and cathode are in synchronism with the electricpulses applied between the electric target and anode.

10. A process as claimed in claim 9, wherein the said electric pulsesapplied to the anode and cathode overlap the pulses applied to the anodeand cathode overlap the pulses applied to the target and anode in timedsequence.

11. A process as claimed in claim 9, wherein a magnetic 5 6 eld isapplied to the discharge between the anode and the OTHER REFERENCEScathode by a source of magnetic flux.

12. A process as claimed in claim 11, wherein said P 'g'cl'gphcatlon ofBerghaus et al, SBI. N0. 283,312, magnet field is moved during thedeposition sequence to HG' s* d t l J f A l. d Ph l 33 N spread themolecular discharge over a uniform area. 5 10 cnberslgr; 1;"299 2 pp leyslcs VO 1 0 13. A process according to claim 8 in which the targetmaterial is a monomer gas and the material deposited is F' Vramy et al':I of the Electrochemical Soc" Vol' 112, No. 5, May 1965, pp. 4844189.

a polymer.

References Cled ROBERT K. MIHALEK, Primary Examiner UNITED STATESPATENTS 1o 3,021,271 2/1962 Wehner :m4-29s US- C1- X-R- 3,347,77210/1967 Laegreid et al 204-298 204-298

